Heat oven to 375°F.
Butter a 9-inch square pan.
For the cake:
In a large bowl, beat softened butter and sugar until blended. Add milk and egg.
In another bowl, combine flour, baking powder and salt. Add to butter mixture and mix just until combined. The batter will be very thick.
Carefully fold in blueberries.
Scoop the mixture into buttered pan and gently press it out evenly.
For the crumble topping:
Combine all topping ingredients in food processor or use a fork/fingers to combine until crumbles (large and small) form. Sprinkle evenly over batter.
Bake at 375°F for 45 to 50 minutes or until toothpick inserted in center comes out clean.
Cool 20 to 30 minutes before serving.